Correct syntax for "mirror" pie operator in UV Editor as one operator
The description of the operator can be found here.

bpy.types.UILayout.operator returns an bpy.types.OperatorProperties object.

So if you need to assign multiple presets, assign it to a local variable first

op_props = pie.operator("transform.mirror", text="Mirror")
op_props.constraint_axis = True, False, True
#...


When the operator's invoke method is called the operator may use user input to set its properties or run modal, before finally running its execute method.

To skip the invocation you might try:

#...
pie.operator_context = "EXEC_DEFAULT"
pie.operator("transform.mirror").constraint_axis = True, False, True

pie.operator_context = "INVOKE_DEFAULT"
#...


See UILayout.operator_context for the available options